Optical fiber connectors are an essential part of substantially any optical fiber communication system. For instance, such connectors are used to join segments of fiber together, to connect fiber to active devices such as radiation sources, detectors and repeaters, and to connect fiber to passive devices, such as switches, multiplexers, and attenuators.
A typical optical fiber connector comprises housing and a ferrule assembly within the housing. The ferrule assembly comprises a ferrule, which has one or more fiber channels to accommodate fibers, and a fiber secured in each channel such that the end of the fiber is presented for optical coupling by the ferrule. The housing is designed to engage a “mating structure” having an optical path to which the fiber optically couples during mating. The mating structure may be another connector or an active or passive device as mentioned above. The optical path may be, for example, a fiber in a ferrule, a waveguide in a substrate, a lens, or an optically-transparent mass. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially and laterally aligned with optical pathway of the mating structure. This way, light from the fiber is optically coupled to the optical pathway. If the cores of the mating fibers are not precisely aligned, the optical coupling suffers attenuation.
Of particular interest herein, are ferrules having alignment features such as the mechanically transferrable (MT) ferrule 200, shown in FIG. 2. The MT ferrule 200 is generally used for multi-fiber applications and thus has a row 201 of fiber channels 202, and is characterized by alignment pin channels 203 and 204 defined at each end of the front face of the ferrule. Alignment pins (not shown) are inserted in the alignment pin channels of mating ferrules such that the ferrules mate in a precise orientation, thus ensuring that the cores of fibers held in the mating ferrules are aligned.
To ensure proper alignment, the alignment pin channels are positioned to minimize angular misalignment. Specifically, referring to FIG. 2, the alignment pin channels 203, 204 have centers 203a, 204a, respectively, which are aligned along an axis 205. It is important that the axis be as close as possible to parallel with the row 201 of fiber channels 203. (In the ferrule 200 shown in FIG. 2, the row is coincident with the axis 205, although in multiple row MT ferrules this would not be the case.) Having the axis 205 parallel to the row 201 is an important parameter to ensure that the fiber cores of mating ferrules are aligned. By way of contrast, if the alignment pin channel 204′ were offset along the y axis as shown (greatly exaggerated), the axis 205′ joining the centers 203a and 204a forms an angle α with the row 201. This angle α is referred to herein as angular misalignment.
The tolerance for angular misalignment of MT ferrules has become more stringent as the use of the MT ferrule has evolved. The MT ferrule was initially introduced as a multi-mode ferrule. A multi-mode fiber has a relatively large core diameter of about 50 microns, and thus aligning the cores of mating ferrules it relatively simple compared to single mode applications. Additionally, because multi-mode applications were usually short-distance applications, relatively high loss, for instance, about 1 dB, could be tolerated. Consequently, the tolerances of a multi-mode ferrule were relatively relaxed, allowing an angular misalignment of about one degree.
As the MT ferrule evolved and was applied to single mode applications, the angular tolerance was reduced. Specifically, a single mode fiber has a relatively narrow core of about 9 microns. Accordingly, any misalignment of a single mode fiber results in significant attenuation. Additionally, given the long range applications in which single mode fibers applications are typically used, very little loss can be tolerated, for example, less than 0.2 dB. Therefore, for the MT ferrules to accommodate this low loss requirement, the angular misalignment tolerance of a single mode MT connector is about 0.5 degrees. This is significantly less than the multi-mode application. Furthermore, in certain “low loss” single mode applications this angular misalignment is further limited to just 0.2 degrees.
Despite the exacting angular misalignment tolerance and other tight tolerances of the single mode MT ferrules, Applicants have discovered several problems with their performance. In particular, performance repeatability of mated MT single mode ferrules varies considerably. For example, a particular optical coupling in mated single mode MT ferrules may be satisfactory in one instance, but when the ferrules are re-mated, its performance may drop considerably. Additionally, Applicants have discovered that the optical coupling performance between fibers varies considerably during thermal cycling of single mode MT ferrules. For example, referring to FIG. 5, the attenuation of a given optical coupling varies with temperature cycling between 60 and −10° C. Moreover, Applicants discovered an initial jump in attenuation during the initial temperature cycle. The optical coupling's performance never recovered from this initial jump, even after the temperature was lowered.
Therefore, what is needed is an MT ferrule suitable for single mode applications that provides consistent optical performance over repeated mating cycles and thermal cycles. The present invention provides this need among others.